RESONANCE RAMAN STUDIES OF BENZENE-DERIVATIVES WITH METHOXY SUBSTITUTION - CONFORMATIONAL SYMMETRY-BREAKING EFFECTS

Resonance Raman intensities show that methoxy substitution induces con siderable allowed character to the L-a transition of benzene. In symme tric 1,3,5-trisubstituted benzene derivatives the induced allowed char acter should cancel by symmetry. However, in 1,3,5-trimethoxybenzene t he resonance Raman spectrum shows a strong fundamental transition of v (8), the clear signature of an allowed transition. It is shown that th is is due to the presence of a C-s conformer of 1,3,5-trimethoxybenzen e in which one of the three methoxy groups is rotated relative to the other two. This is supported by ab initio calculations of conformation al energy, induced transition moment and the splitting of the v1 band. (C) 1997 by John Wiley & Sons, Ltd.